Home / Processes
How Vacuum Grade Parts Are Machined, Finished and Cleaned
A chamber lid or a gas block passes through eight steps after the stock is cut, and most of the schedule risk sits after the last tool change. This is the route, with what each step needs from your drawing.
The route a vacuum part takes
Not every part needs every step. A bracket for the atmosphere side stops after inspection. A wetted gas block goes through all eight, and the last three decide whether the fab accepts it.
| Step | What happens | What the drawing has to say | Where it usually goes wrong |
|---|---|---|---|
| Material release | Plate or bar bought to grade. The heat lot is recorded and travels with the part. | Alloy, temper, certificate type and any restricted material list. | A stockist certificate in place of the mill certificate, or a different temper. |
| Rough and rest | Bulk removal, then a rest or a thermal stress relief before the finish cuts on large plates. | Flatness callouts and a datum scheme that matches how the part is used. | Skipping the rest step. The plate moves and the flatness drifts after delivery. |
| Finish machining | Sealing faces, O-ring grooves, bores and threads cut to final size. | Ra per face, groove standard, thread spec, vented or not. | One Ra value in the title block, applied to every face. |
| Welding | TIG or electron beam on stainless. On UHV parts the joint is made from the vacuum side with full penetration. | Joint design, weld standard, leak rate after welding. | Outside welds that leave a trapped volume behind the joint. |
| Deburr and pre-clean | Burrs removed, cutting fluid stripped before the part goes to any finishing line. | Edge break, no silicone, approved coolant if restricted. | Burrs left at the intersection of cross drilled gas passages. |
| Finishing | Anodize, electroless nickel, electropolish, passivation or bead blast, usually at a specialist line. | Specification, class, thickness, masking map, dimensions before or after coating. | No masking drawing, so threads and sealing faces get coated. |
| Inspection | CMM, surface roughness, flatness and leak test where called out. | Which features are reported and at what sample size. | A report that covers only the features the shop marked as critical. |
| Precision clean and pack | Ultrasonic wash, DI water rinse, dry, then bag in a clean area. | Cleaning spec, bag material, purge, label position. | Generic packing on a part that was clean when it left the line. |
Which process makes which feature
Typical capability, read as a range shops usually work in rather than a promise on your part. The shop that quotes confirms what it holds on your geometry and material.
| Feature | Usual process | Typical capability | Note for the drawing |
|---|---|---|---|
| Large sealing face on a chamber body | Face milling with a large cutter, light finish pass, part unclamped between passes | Flatness 0.02 to 0.05 mm over 500 mm, Ra 0.4 to 0.8 um | Say whether flatness is checked free or restrained. |
| O-ring groove, rectangular or dovetail | Contour milling, dovetail form cutter | Groove size to the seal maker table, Ra 0.4 to 0.8 um on the seal contact | Radial tool marks across the seal path cause leaks. Ask for circumferential lay. |
| CF knife edge | Turning, then protected right after the cut | Edge geometry per ISO 3669 flange dimensions | Edge damage in handling is a common reject. Caps go on at the machine. |
| Showerhead gas holes | Small diameter peck drilling, EDM drilling for the smallest holes | Hole count in the hundreds or thousands, consistent diameter and clean exits | Drill wear across the run changes flow. Ask how tools are changed. |
| Cross drilled gas passages | Deep hole drilling, plugged or welded access holes | Long length to diameter ratios, intersections deburred | Electropolish after machining only works on surfaces the electrolyte reaches. |
| Wafer contact or chuck face | Surface grinding, lapping | Flatness under 0.01 mm on smaller parts | Measure on a granite reference at controlled temperature. |
Flatness and Ra apply to the feature that carries them. A part where every face is held to the sealing face value costs more on every piece.
Finishing options and what they do to the part
Each one changes the dimensions, the surface chemistry or both. Name the specification, the class and the thickness, not just the process.
- Type III hard anodizeMIL-A-8625 Type III. Typical thickness 25 to 50 um, about half grows outward and half goes into the metal. Used on plasma facing and wear surfaces of 6061 and 5083. The seal method is a callout: hot DI water, unsealed or a metal salt seal behave differently in a chamber, and many process chamber specs exclude metal salt seals.
- Type II sulfuric anodizeMIL-A-8625 Type II, typically 5 to 25 um. Corrosion protection and colour on atmospheric side hardware. Not a plasma barrier.
- Electroless nickelAMS 2404 or ASTM B733. Even thickness inside bores and pockets, typically 5 to 25 um. Mid phosphorus (5 to 9 percent) for wear, high phosphorus (10 percent and up) for corrosion. Many process specs keep nickel off plasma and gas wetted surfaces because of metal contamination, so it is more common on mechanisms, frames and atmosphere side parts.
- ElectropolishAnodic removal of the smeared surface layer on stainless. Lowers Ra and raises the chromium content of the surface. ASTM B912 covers passivation by electropolishing. Wetted 316L in gas systems commonly calls out Ra 0.25 um or better, with surface inspection along the lines of SEMI F19.
- PassivationASTM A967 or AMS 2700, nitric or citric acid. Removes free iron from stainless with no dimensional change. Often called out together with electropolish.
- Bead blastA uniform matte finish, or the surface preparation under a thermal spray coating. Media type and grit belong on the drawing, and so does the cleaning after it, because loose media is a particle source.
- Thermal spray coatingsYttria or alumina on etch parts, arc spray texture on PVD shields. Done by coating houses. The machine shop prepares the surface, masks and protects the edges the coating must not reach.
More on the coating side: plasma resistant coatings on etch parts and electropolishing 316L for gas parts. Background on the anodic process: anodizing.
Precision cleaning is a process, not a courtesy
A typical sequence for parts that go under vacuum. Your cleaning specification replaces it where one exists, and it should be on the drawing so it is quoted.
Degrease
Cutting fluid and handling oils removed with a solvent or alkaline degreaser.
Ultrasonic wash
Alkaline detergent bath, part fixtured so blind holes drain and cavitation reaches internal faces.
DI water rinse
Cascade or spray rinse in deionised water. The resistivity of the final rinse is the number worth asking about.
Dry
Filtered nitrogen or clean dry air, then an oven where water can hide in threads or passages.
Bake, if specified
A vacuum or air bake to drive off water and light organics before packing. Only when the drawing asks.
Double bag and label
Inner bag closed in the clean area, outer bag for transport, label on the outer bag only.
The rules behind it: cleanliness and surface finish for vacuum parts.
Tests that sit inside the route
Ask for the ones you will read. Each is a line item.
- Helium leak testMass spectrometer test with helium as tracer, by spray, hood or bagging. UHV components are commonly specified around 1 x 10-9 mbar L/s or tighter. ISO 20485 describes tracer gas methods. See helium leak testing chamber parts.
- CMM reportFull or partial dimensional report against the drawing revision. Name the features and the sample size. See CMM and flatness reports.
- Surface roughnessProfilometer reading on the faces that carry an Ra callout, with the cutoff length stated when it matters.
- RGA or outgassingResidual gas analysis after bake, usually on UHV hardware or a first article. Done by the shop or an outside lab.
- Particle or wipe testLiquid particle count or a wipe test after cleaning when the drawing names a cleanliness level.
Design notes that lower the price
- Vent blind tapped holes, or call out vented screws. A trapped volume behind a screw is a virtual leak. See vented screws and vacuum fasteners.
- Give Ra only where the function needs it. Ra 0.8 um comes off the cutter, Ra 0.4 um and better needs a finishing pass and inspection time.
- Design welds to be made from the vacuum side with full penetration, and leave stock for machining after welding where distortion matters.
- Keep internal passages reachable by the cleaning and electropolish media. A passage the electrolyte cannot reach keeps its machined surface.
Questions about the process route
Should tolerances be stated before or after anodize?
State which one you mean. Type III hard anodize grows each surface by roughly half the coating thickness, so a bore loses about the full thickness on its diameter. For fits, most drawings dimension after coating and give the coating thickness so the shop machines undersize. Threads and sealing faces are usually masked.
Is electropolish the same as passivation?
No. Passivation to ASTM A967 is a chemical treatment that removes free iron and leaves the dimensions alone. Electropolishing removes metal, lowers Ra and also passivates the surface, which is what ASTM B912 covers. Gas wetted 316L usually gets electropolish, structural stainless usually gets passivation only.
Can one shop run all of these steps?
Machining, deburring, inspection and cleaning are usually in house. Anodize, plating, electropolish and thermal spray are usually done by specialist lines the shop buys from. Ask which steps are outside processes and who runs them. That answer explains most of the lead time.
What leak rate should I put on the drawing?
The rate your system needs, with the units, the test method (spray, hood or bagging), the tracer gas and whether it applies to the component or the assembly. Without the method a shop cannot price the test, and two shops will test two different ways.
Send the drawing with the route on it
Name the finish, the cleaning and the tests in the RFQ. We match it with up to 3 shops whose finishing chain fits and reply within 24 business hours.
- MaterialsGrades, designations and certificates for vacuum parts.
- ApplicationsParts by tool module, from etch to EFEM.
- Qualifying a shopWhat to ask before the first order.